1. Field of the Invention
The present invention relates to a motor control device and an electric power steering system.
2. Description of Related Art
A conventional motor control device has a phase opening device that is disposed on a power line between a motor and an inverter circuit and is used to interrupt current. For example, Japanese Patent Application Publication No. 2013-17304 (JP 2013-17304 A) discloses a motor control device that controls a three-phase (U, V, and W phases) brushless motor that generates assist force. This motor control device controls electric power supplied to the three-phase brushless motor, and performs assist control that provides assist force. This motor control device has phase opening relays (FETs: field-effect transistors) that are disposed in respective three-phase power lines and prevent the flow of a current into the motor. In the event that a certain failure occurs, the motor control device opens these phase opening relays. With the three-phase power lines interrupted in this manner, the current flow into the motor is prevented.
When one of the three phase-opening-relay FETs has a short-circuit failure, the assist control can be continued with the remaining two phase-opening-relay FETs. However, the phase with the short-circuit failure causes current to flow through the motor coils to the other two phases, which deteriorates accuracy of the assist control performed by the remaining two phases. When one of the phase opening relays has a short-circuit failure and then the remaining two phase opening relays also fail due to short-circuits, or when an FET of the inverter circuit fails, the assist control has to be stopped to prevent current flow into the motor. Such a problem can occur, not only in the assist control but also in controlling the motor for the other purpose.